GNUForth

Gforth is a fast and portable implementation of the ANS Forth language. It works nicely with the Emacs editor, offers some nice features such as input completion and history, backtraces, a decompiler and a powerful locals facility, and it has a comprehensive manual. Gforth combines traditional implementation techniques with
newer techniques for portability and performance: its inner interpreter is direct threaded with several optimizations, but you can also use a traditional-style indirect threaded interpreter. Gforth is
distributed under the GNU General Public license.